This invention relates to diphenyl ether compounds expressed by the general formula ##STR4## wherein X represents a hydrogen atom or a nitro group, and also to a process for preparing the same and diphenyl ethers expressed by the general formula ##STR5## wherein R represents a hydrogen atom, or substituted or unsubstituted alkyl group, alkenyl group or alkynyl group, and Y represents an oxygen atom, a sulfur atom, a nitrogen atom or an imino group, characterized by reacting 2-chloro-4-trifluoromethylphenyl-3,4-dinitrophenyl ether with a compound expressed by the general formula EQU RYH (III)
wherein R and Y have the same meanings as defined above, respectively.
A number of compounds as expressed by the general formula (II) are known to show the excellent herbicidal activity.
For instance, Japanese Laid-Open patent application Nos. 49-236, 50-37740 and 51-79721 deal with processes of preparing compounds of the just-mentioned type. Then, reference is made particularly to the process of the invention, so as to illustrate the preparation of the compounds expressed by the foregoing general formula (II).
(A) Preparation Process Described in Japanese Laid-Open Patent Application No. 49-236
2,4-Bis(2-chloro-4-trifluoromethylphenoxy)benzene is obtained by condensation reaction of 3,4-dichlorobenztrifluoride with resorcinol and then nitrated to give 2,4-bis(2-chloro-4-trifluoromethylphenoxy)nitrobenzene. The nitrated product is reacted with alcohols, thiols or amines under alkaline conditions to allow interchange reaction of the phenoxy group in ortho position to the nitro group, whereby an intended compound is obtained.
(B) Preparation Process Described in Japanese Laid-Open Patent Application No. 50-37740
2-Chloro-4-trifluoromethylphenyl-3-chlorophenyl ether is obtained by condensation reaction of 3,4-dichlorobenztrifluoride with metachlorophenol and then nitrated to give 2-chloro-4-trifluoromethylphenyl-3-chloro-4-nitrophenyl ether. The nitrated product is reacted with thiols under alkaline conditions to allow interchange reaction of the chlorine atom in ortho position to the nitro group, whereby an intended compound is obtained.
The two production processes which are considered to be the prior art of the invention have been described but these processes are not satisfactory and involve various problems to be solved. The process (A), for instance, requires 2 moles of 3,4-dichlorobenztrifluoride to obtain one mole of the active compound. In order to effectively utilize the starting compounds, the 2-chloro-4-trifluoromethylphenol produced in the final interchange reaction must be recovered for reuse, which results in complicate reaction process and increased production cost.
With the process (B), on the other hand, the nitration reaction of 2-chloro-4-trifluoromethylphenyl-3-chlorophenyl ether inevitably involves contamination of the intended product with its isomer of 2-chloro-4-trifluoromethylphenyl-2-nitro-5-chlorophenyl ether. In addition, since the 2-chloro-4-trifluoromethylphenyl-3-chloro-4-nitrophenyl ether product remains liquid at a normal temperature, it is difficult to separate the isomer having a very similar properties from the product, thus lowering the purity of a final product.